Absorption Spectroscopy has long been a tool in the analytical chemists' repertoire of analytical techniques. The fundamental theory is based on the observation that atoms and molecules absorb energy from electromagnetic radiation of a particular wavelength, typically, but not exclusively in the form of photons passing through a sample. As each atom or molecule has a distinctive pattern of absorption wavelengths it is possible to deduce the atomic or molecular species under analysis. It is also possible to obtain a measure of the concentration of the species by means of reference to a calibration. The Beer-Lambert Law quantitatively relates the reduction in intensity of electromagnetic radiation of a particular wavelength to the concentration of absorbers (atoms or molecules) of that wavelength and the distance travelled by the electromagnetic radiation through the sample, by the expression:Id=Ioe−Nβd  Eqn1where N=the number of absorbers per unit volume.                β=the absorption coefficient.        Io=the initial intensity of the source.        Id=the measured intensity after passing through the sample.        d=the distance travelled through the sample.        
This can be rearranged as follows:                     N        =                              1                          β              ⁢                                                           ⁢              d                                ⁢                      Ln            ⁡                          (                                                I                  o                                                  I                  d                                            )                                                                    Eq                      n            _                          ⁢        2            Conventional Absorption Spectroscopy has a number of limitations. Although theoretically very sensitive and potentially quantitative, practical limitations imposed by current instrumentation and the method of measurement seriously restrict these capabilities and hence limit the application of absorption spectroscopy. One of these limitations is sensitivity of the measurement technique. From analysis of eqn. 1 it can be shown that there is an optimal range of values for the exponential term βNd. It is then clear that for a given value of β and in order to maintain the optimal value of the expression a reduction of the number of absorbers N must be compensated for by a corresponding increase in the value of d. Further analysis shows that for very small values of N, d can become impractically large. Long path lengths are achieved by means of folded geometries which are described in more detail later. It is also possible to deduce an effective dynamic range for a given instrumental configuration from eqn 1.
Further practical limitations in electromagnetic source stability over both short and medium term time scales and limitations in detection systems impose further limitations on sensitivity and accuracy of measurements. From Eqn 2 it can be demonstrated that there is a need for high accuracy of measurement in both I0 and Id. Since radiation sources have limited stability as stated previously, for the most accurate measurement both I0 and Id must be measured simultaneously. This introduces additional cost and complexity to instrumentation. Both limitations described have partial solutions that are implemented in current instrumentation.
Further limitations become apparent in issues of quantification and reproducibility of measurements. These limitations arise from a combination of some of the previously described practical limitations and from the two point measurement system using I0 at the source and Id at the end of the propagation path through the sample.
Many techniques have been devised with the objective of improving the performance of absorption spectroscopy. These include the use of folded path analysis cells that extend the optical path through the sample and thereby improve sensitivity, the use of detector arrays for spectroscopic analysis and the use Cavity Ringdown Laser Absorption Spectroscopy.
A paper by J. U. White in J. Opt. Soc Am, Vol 32, pp 285-288 describes a geometrical arrangement for extending the pathlength of a light source through an absorption cell by use of multiple reflections and traversals through the sample volume, thereby achieving an extended optical path in a small volume. This type of geometry has been widely adopted in absorption spectroscopy and is referred to as either a White cell or more generically as a folded path geometry cell. FIG. 1 of the accompanying drawings shows a typical folded path cell geometry. A light source 11 produces a light beam 12 which enters the cell 13 through a window 14 that is optically transparent at a wavelength of interest. The light beam 12 within the cell 13 is allowed to traverse the cell until it is incident upon mirror 15 where it is focused and redirected as a beam 16 towards a second optically transparent window 17 and thence to the detector 18. The main advantage of this geometrical arrangement is to increase the pathlength through the cell thereby increasing the sensitivity of the absorption measurement, as previously discussed. Many variations on this type of geometry have been devised including multiple reflection types. All have at least one common feature; that is, detection is carried out at the end of the propagation path of the light beam through the sample.
Another example of a multiple-pass cell is described in U.S. Pat. No. 5,220,402. As shown in FIG. 2 of the accompanying drawings, this arrangement comprises a focused light source 21 that is directed into a circular cell 22 through a wedge-shaped lens 23 that also acts as a window into the cell. The internal structure of the cell 22 is such that it provides some focusing of the light in both axial and radial planes giving a focal point close to the axis of the cell. By way of illustration, FIG. 2 shows one of a plurality of paths that the light might follow inside the cell. Light entering the cell through the lens 23 is directed to be incident on a reflecting surface 24 of the cell where it undergoes a reflection 25 directing it back through the axial region of the cell to a farther reflection point on the housing wall. This process continues until the light exits the cell through a further lens 26 where it is directed to a means of measurement 27. The primary aim is to maximise the optical path length within a cell of given volume, and both two and three dimensional geometries are described.
U.S. Pat. No. 5,485,276 describes another arrangement for increasing the optical path length of light within a gas absorption cell using multiple reflections and a collimated beam. As shown schematically in FIG. 3, this arrangement comprises a diode laser 30 producing a collimated beam 31 which enters a gas absorption cell 32 through an aperture or window 33. The collimated beam 31 inside the cell 32 is so directed that it is incident upon a mirror 35 on the opposite side of the cell 32. The mirror is angled so that the reflected beam 36 is then directed towards a further mirror 37 positioned on the same side as the entrance aperture, but displaced from it. During traversal between mirrors, the light suffers a loss in intensity due to absorption by absorbing fluid in the cell. Further reflections and traversals continue in a like manner until the light exits the cell at an exit aperture 38 whereupon it is detected by a detector 39. The mirrors are all contained within a two dimensional plane.
In general, these folded path geometries have been used for the purpose of increasing the path length of the light through a sample in order to improve measurement capability for low concentrations in absorbance spectroscopy.
It is also known to use multiple detectors in spectroscopy, primarily in detector arrays used for measuring spectra. For example, U.S. Pat. No. 5,721,430 describes an apparatus that uses multiple detectors in an NDIR analayser and a wideband light source. These multiple detectors are configured to operate in parallel at the end of the optical pathlength, and the detectors are provided with individual optical bandpass filters enabling them to detect different wavelengths.
Another arrangement using multiple detectors at the end of the propagation path through the sample is described in U.S. Pat. No. 5,854,684. In general, multiple detectors have been used for the purpose of measuring different wavelengths in a spectroscopic instrument after either an optical filter system or the application of a wavelength dispersive device.
A recent development in the field of high sensitivity optical spectroscopy is Cavity Ringdown. The technique of Cavity Ringdown is derived from the high finesse optically resonant cavities used in laser technology and uses the principle of a highly resonant optical cavity in the measurement of low concentrations of gases. In Cavity Ringdown the laser light is introduced into the cavity from an external laser source. Cavity Ringdown has its roots in a paper by J. M. Herbelin et al published in the J. Appl. Opt. 19(1) p 144-147 in 1980 in which he describes the measurement of the reflectivity coefficient of high performance mirrors using a cavity attenuated phase shift (CAPS) technique. The mirrors being measured form the ends of the optically resonant cavity. In the described CAPS technique the cavity decay time (deduced from the induced phase shift) is used to calculate the reflectivity coefficients of the mirrors. In the paper, concerns were raised about the potential distortion of the calculations of the coefficients by small quantities of absorbing contaminant gases in the optically resonant cavity. It was further noted that the CAPS technique may have application in the measurement of the concentrations of small quantities of gas deliberately introduced into the optically resonant cavity. A paper by D. Z. Anderson et al published in Appl. Opt. 23(8) p 1238 in 1984 describes a further development in which the decay of light intensity with time is directly observed.
The advent of high performance, fast pulsed lasers obviates the need for attenuated phase shift measurements, and a paper by A. O'Keefe et al in Rev Sci Inst, 1988, 59(12), p2544 describes the use of pulsed lasers and the measurement of decay in intensity with time of the pulse intracavity. All subsequent developments in cavity ringdown techniques are based on variations of this basic approach. A review of the work of Herbelin et al and of the subsequent developments is contained in a paper by J. J. Scherer et al in Chem Rev 1997, v 97, pp25-51.
A typical cavity ringdown apparatus, is now described by reference to FIGS. 4a and 4b. The apparatus consists of laser 40, a high finesse resonant optical cavity 41, a photon mulitplier 42, an amplifier 43, an oscilloscope 44 and a computer 45. The optical cavity consists of an outer housing 46, typically a cylinder, and two high reflectivity concave mirrors 47,48 that are additionally used to seal the ends of the cavity 41. The mirror 47 nearest the laser 40 is called the entrance mirror and the mirror 48 at the other end the exit mirror. The mirrors 47,48 have a coefficient of reflectivity, R, which is typically of the order of 0.995 or higher. The cavity contains an absorbant gas species for analysis by absorption of photons.
The oscilloscope 44 is used to digitise the amplified signal from the photon multiplier 42 and the computer 45 is used for general control of the timing electronics and for recording the digitised output from the oscilloscope.
In a typical cavity ringdown experiment, a fast (of the order of nanoseconds) pulse of laser energy of a known wavelength is focused and directed into the optical cavity through the entrance mirror 47. A small amount of the laser energy equal to (1-R), is coupled into the cavity and the rest of the laser energy is reflected back from the mirror and has no further function in the measurement. The light in the cavity is now trapped and reflects back and forth between the two mirrors 47,48. A small fraction (1-R) of the trapped laser energy passes through each mirror at each reflection. By measuring the small component (1-R) of the light that is transmitted through the exit mirror 48 after each traversal through the cavity as a function of time t, a measure of the decay of the light pulse can be made. This decay with time I(t) is due a combination of reflection losses and absorption by the gas contained in the cavity. This measured intensity can be shown to be proportional to the losses in the cavity where,I(t)∝Rtote−σ(λ)Nt  Eqn3Equation 3 has the form of the Beer-Lambert law which relates the losses due to absorption to the number of absorbers present in the cavity, but is modified to allow for the additional losses due to multiple reflections, where Rtot is the total loss coefficient due to the reflections. A typical decay curve is shown in FIG. 4b. By calibrating the apparatus with no absorbing gas present, a value for Rtot due solely to reflection at the mirrors can be determined. This value can then be taken into account in the measurement of the decay when an absorbing gas is present, and the number of absorbers determined. Decay times for a typical cavity ringdown measurement are usually in the region of the one to tens of microseconds, in part dependent upon the cavity length and also on the concentration of absorbers present.
A drawback of the cavity ringdown technique is the requirement to measure the decaying intensity of electromagnetic radiation as a function of time, typically over a time interval which is only of the order of tens of microseconds. Accordingly, implementation of cavity ringdown techniques is both complex and expensive, requiring the use of fast pulsed lasers, high finesse, high Q optical cavities and high speed digital timing electronics.
The cavity ringdown technique also has the disadvantage that the pulsed electromagnetic radiation undergoes multiples passes through the same sample region, and this may reduce the sensitivity of the measurement being made.
Another major problem associated with the use of cavity ringdown is the poor efficiency of coupling of the electromagnetic radiation into the optical cavity. U.S. Pat. No. 5,815,277 describes a method for alleviating this problem. In this method, an acoustic optical modulator (AOM) is placed inside the cavity in order to redirect the light pulse onto the optical axis of the resonant cavity. This increases the coupling efficiency to in excess of 40%. Although this approach gives significant improvements in efficiency, it does introduce an additional optical component into the cavity which may decrease the effectiveness of both the measurement and its applicability. The AOM adds to the cost and to the complexity of the control system, factors which must be considered against a reduction in the requirements for the initial energy of the laser source and the potential to use lower cost detection methods.
A further recent development in the use of cavity ringdown is a technique known as Intracavity Laser Spectroscopy (ILS) where the laser cavity itself is used as the analysis cell. This type of device has many advantages in terms of size and sensitivity, but is still limited in its applicability. U.S. Pat. Nos. 5,841,533, 5,742,054 and 5,747,807 describe three examples of the application of this technique.
It is an object of the present invention to provide an apparatus and method for measuring decay in intensity of electromagnetic radiation passing through a radiation-absorbent sample due to absorption by the sample which at least alleviates the above-described short-comings of existing arrangements.